Infra-red reflecting layered structure

ABSTRACT

The invention relates to an infra-red reflecting layered structure comprising a transparent substrate layer; a first metal oxide layer; a first silver containing layer, a second metal oxide layer; a second silver containing layer and a third metal oxide layer. The first, second and third metal oxide layer have a refractive index of at least 2.40 at a wavelength of 500 nm. The layered structure according to the present invention laminated on glass has a visual light transmittance (VLT) higher than 70% and a solar heat gain coefficient (SHGC) lower than 0.44. The invention further relates to the use of a layered structure as a transparent heat-mirror.

FIELD OF THE INVENTION

The invention relates to an infra-red reflecting layered structure andto the use of such a layered structure as heat-mirror.

BACKGROUND OF THE INVENTION

Heat-mirrors that reflect radiation in the infrared spectrum whiletransmitting radiation in the visible spectrum have importantapplications for example as windows in buildings or vehicles.

For transparent heat-mirrors, visual light transmittance must be high,and hence the reflectivity and absorptivity must be low.

In the United States of America for example, automotive windshields musthave a transmittance of visible light of at least 70%.

In the infrared, however, the heat-mirror must have high reflectivityand so transmittance and absorptivity in the infra-red must be low.

Heat-mirrors comprising a stack of alternating dielectric and metallayers are known in the art.

To obtain a heat-mirror characterised by a low heat transmittance,generally at least three metal layers are necessary. However, the numberand the thickness of the metal layers have a negative influence on thevisual light transmittance and on the cost and complexity of themanufacturing process.

It is well known to use silver as metal layer. However, a silver layerhas a low stability, low durability and poor moisture and weatherresistance.

SUMMARY OF THE INVENTION

It is an object of the present invention to avoid the drawbacks of theprior art.

It is another object of the invention to provide an improved infra-redreflecting layered structure.

It is also an object to provide an infra-red reflecting layeredstructure characterised by a good visual light transmittance and a lowsolar heat gain coefficient with a minimum number of metal layers.

It is a further object of the invention to provide an infra-redreflecting layered structure having silver containing layers with a highstability and a high weather resistance.

According to a first aspect of the present invention an infra-redreflecting layered structure is provided. The layered structurecomprises:

-   -   a transparent substrate layer;    -   a first metal oxide layer;    -   a first silver containing layer;    -   a second metal oxide layer;    -   a second silver containing layer and    -   a third metal oxide layer.

The first, second and third metal oxide layer have a refractive Index ofat least 2.40 at a wavelength of 500 nm.

In the layered structure according to the present invention, the numberof pairs silver containing layer—metal oxide layer is limited to two.The thickness of the various metal oxide layers and the thickness of thefirst and second silver containing layers are adapted to each other sothat the layered structure, laminated on glass, has a visual lighttransmittance (VLT) higher than 70% and a solar heat gain coefficient(SHGC) lower than 0.44.

The light to solar gain ratio (LSG ratio) of the layered structurelaminated on glass is preferably higher than 1.60. More preferably, theLSG ratio is higher than 1.65, for example 1.69.

The visual light transmittance (VLT) refers to the percentage of thevisible spectrum (380-780 nm) that is transmitted through a window.

The solar heat gain coefficient (SHGC) is the fraction of incident solarradiation (350-2500 nm) admitted through a window, both directlytransmitted and absorbed and subsequently released inward by means ofconvection and radiation. SHGC is expressed as a number between 0 and 1.The lower a window's solar heat gain coefficient, the less solar heat ittransmits.

The light to solar gain ratio (LSG ratio) is defined as

$\frac{VLT}{{SHGC} \star 100}.$The LSG ratio provides a gauge of the relative efficiency of differentglass types in transmitting daylight while blocking heat gains. Thehigher the ratio, the brighter the room is without adding excessiveamounts of heat.

The metal oxide may comprise any transparent material. However, metaloxide having a high refractive index and an almost zero extinctioncoefficient are preferred.

Therefore, in optical coatings where the optical thickness of the layersis of importance, the physical thickness of metal oxide having a highrefractive index can be kept lower than the physical thickness of metaloxides having a lower refractive index.

The metal oxide layers of the layered structure can be deposited by anytechnique known in the art. Preferred techniques comprise physical vapordeposition techniques such as sputter deposition or chemical vapordeposition techniques.

A preferred metal oxide layer comprises TiO₂ and more particularly TiO₂that is mainly composed of rutile phase and that is very dense. Thistype of TiO₂ has a refractive index of 2.41 at 510 nm.

A TiO₂ layer can be deposited by a reactive sputter deposition processfrom a Ti-target, a TiO₂-target or a substoichiometric TiO_(x)-target(with x between 1.75 and 2).

TiO₂ mainly composed of rutile phase is preferably deposited by DCmagnetron sputtering using a TiO_(x) targets (preferably a rotatableTiO_(x) target) with x between 1.5 and 2, for example between 1.5 and1.7.

These rotatable targets are produced by plasma spraying of rutile powderin a reducing atmosphere (e.g. Ar/H₂) on a stainless steel backing tube.The targets have enough electrical conductivity to be used as cathodesin a DC magnetron sputtering process and can withstand extremely highpower levels. As a result, it is possible to achieve very high sputterdeposition rates, at lower investment cost (both the deposition sourceitself and the power supply are considerably cheaper).

Other metal oxides having a high refractive index are for example BiO₂(refractive index 2.45 at 550 nm) or PbO (refractive index 2.55 at 550nm).

The different metal oxide layers of the layered structure may comprisethe same material or may comprise a different material.

The first and second silver containing layers may comprise pure silver(i.e. silver with unavoidable impurities) or silver in combination withanother element as for example gold, platinum, palladium, copper,aluminium, indium or zinc and/or mixtures thereof.

The silver containing layers comprise for example silver and up to 30 wt% of another element such as gold, platinum, palladium, copper,aluminium, indium or zinc and/or mixtures thereof.

A preferred silver containing layer comprises 10 wt % gold.

The silver containing layers are preferably deposited by a vacuumdeposition technique, for example by sputtering or evaporation.

The deposition of the silver containing layers needs specialprecautions, because

-   -   (i) silver is, although often referred to as a precious metal,        very prone to corrosion, and    -   (ii) the intermixing of the metal oxide and the silver layers        has to be avoided: absorptance is essentially proportional to        η.k; hence the presence of a rather thick mixed TiO2 (high η)—Ag        (high k) layer will seriously increase the total absorptance of        the layered structure and can eat away a big part of the        theoretically achievable visual light transmittance.

This means that it can be preferred that the silver containing layerand/or the interface between the silver containing layer and the metaloxide layer is specially protected. This can for example be achieved bymeans of an intermediate layer between the metal oxide layer and thesilver containing layer; between the silver containing layer and themetal oxide layer or by means of an intermediate layer on both sides ofthe silver containing layer.

Such an intermediate layer preferably comprises gold, for example puregold (i.e. gold with unavoidable impurities) or gold in combination withup to 30 wt % of another element such as silver.

The intermediate layer has preferably a thickness between 0.5 and 10 nm,for example 1 nm.

Preferably, the intermediate layer is deposited by sputter deposition.

The layered structure according to the present invention comprises atleast one transparent substrate layer.

The transparent substrate layer or layers may comprise a glass layer ora plastic layer for example a plastic layer made of polycarbonate,polyacrylate, polyester such as polyethylene terephtalate (PET),cellulose tri acetated (TCA or TAC) or polyurethane.

Possibly, an additional layer is deposited on top of the layeredstructure. Such an additional layer comprises for example a protectivelayer or an abrasion resistant layer.

According to a second aspect of the invention, the use of an infra-redreflecting layered structure as a transparent heat-mirror is provided.

According to further aspects a method of reducing the number of silvercontaining layers in an infra-red reflecting layered structure and amethod of improving the visual light transmittance of an infra-redreflecting layered structure are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIGS. 1, 2 and 3 show different embodiments of an infra-red reflectinglayered structure according to the present invention.

FIG. 4 shows the optical properties of a TiO₂ coating.

FIG. 5 shows the cross-section of a spectrally selective solar controlwindow film.

FIG. 6 shows the cross-section of an automotive glazing comprising alayered structure according to the present invention.

FIG. 7 shows the transmittance of a layered structure according to thepresent invention.

FIG. 8 shows the reflectance of a layered structure according to thepresent invention.

FIGS. 9 and 10 compares the transmittance and the reflectance of alayered structure according to the present invention with two othertypes of layered structures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

An embodiment of an infra-red reflecting layered structure 10 is shownin FIG. 1. The layered structure comprises three metal oxide layers 12,14, 16 and two silver containing layers 13,15.

The metal oxide layers comprise TiO₂.

The TiO₂ is obtained by DC magnetron sputtering using rotatable ceramicTiO_(x) targets with x between 1.5 and 1.7. These targets have enoughelectrical conductivity to be used as cathodes in a DC magnetronsputtering process.

In FIG. 4, the refractive index (η) and the extinction coefficient (∈)of a TiO₂ coatings can be seen. The refractive index (η) in function ofthe wavelength is given by line 44; the extinction coefficient (∈) of Infunction of the wavelength is given by line 42.

For wavelengths higher than 395 nm, the coating is absorption free. Therefractive index at 510 nm is 2.41, which corresponds to the rutilephase of TiO₂.

The silver containing layers 12, 14 comprise pure silver (i.e. silverwith unavoidable impurities).

In an alternative embodiment the silver containing layers 12, 14comprise a silver layer comprising 10 wt % gold.

The first metal oxide layer 12 and the third metal oxide layer 16 have athickness ranging between 25 and 35 nm.

The second metal oxide layer 14 has a thickness between 50 and 70 nm.

The first and second silver containing layer 13, 15 have a thicknessbetween 10 and 25 nm.

FIG. 2 shows another embodiment of an infra-red reflecting layeredstructure 20. The layered structure is the same as the layered structureshown in FIG. 1 but additionally comprises intermediate layers 27, 27′,respectively between the first silver containing layer 22 and the secondmetal oxide layer 24 and between the second silver containing layer 25and the third metal oxide layer 26.

The intermediate layers comprise gold and have a thickness of 1 nm.

The intermediate layers increase the stability and durability of thesilver containing layers and avoid the intermixing at the interface ofthe silver containing layer and the metal oxide layer.

FIG. 3 shows a further embodiment of an infra-red reflecting layeredstructure 30. Intermediate layers 37, 37′ and 39, 39′ are deposited onboth sides of the silver containing metal layers 33, 35.

The intermediate layers comprise gold or gold comprising 10 wt % silver.

The intermediate layers have a thickness of 1 nm.

FIG. 5 shows the cross-section of a spectrally selective solar controlwindow film 50 comprising:

-   -   a hard coat top layer 52 for example comprising a cross-linked        acrylate;    -   a first PET film 53 having a thickness of for example 23 μm;    -   a layered structure 54 according to the present invention;    -   a first adhesive layer 55;    -   a second PET film 56 having a thickness of for example 23 μm;    -   a second adhesive layer 57;    -   a glass layer 58.

FIG. 5 shows the sequence of the different layers. The thickness of thedifferent layers is not in proportion to the real thickness.

FIG. 6 shows the cross-section of an automotive glazing comprising:

-   -   a first glass layer 62;    -   a first adhesive layer 63 for example comprising a PVB layer        having a thickness of 375 μm;    -   a PET film 64 having a thickness of for example 50 μm;    -   a layered structure 65 according to the present invention;    -   a second adhesive layer 66 for example comprising a PVB layer        having a thickness of 375 μm;    -   a glass layer 67.

The optical properties of the spectrally selective solar control windowshown in FIG. 5 are given in Table 1.

TABLE 1 Visual properties VLT Visual Light Transmittance (%) 71 VLRVisual Light Reflectance (%) 9 Solar Properties SHGC Solar Heat GainCoefficient 0.42 TSER Total Solar Energy Reflected (%) 58 LSG ratiolight-to-solar-gain ratio 1.69 UV properties TUV UV Transmittance (%)<0.2

The transmittance T (expressed in %) of the spectrally selective solarcontrol window film as shown in FIG. 5 is given in FIG. 7 for the UV,visible and near infra-red.

The reflectance R (expressed in %) of the spectrally selective solarcontrol window film as shown in FIG. 5 is given in FIG. 8. Thereflectance is measured on the glass side (line 82) and measured on thefilm side (line 84).

This infra-red reflecting structure according to the present inventioncombines a high visual light transmittance (VLT), with a low visuallight reflectance and with a low solar heat gain coefficient (SHGC). Thestructure is furthermore characterized by a neutral color.

Infra-red reflecting layered structures known in the art need threesilver containing layers to obtain the desired low solar heat gaincoefficient. The layered structures according to the present inventionhave a low solar heat gain coefficient with only two silver containinglayers. This reduced number of silver containing layers has a positiveinfluence on the visual light transmittance.

In FIGS. 9 and 10, the transmittance and reflectance of the spectrallyselective solar control window film as shown in FIG. 5 is compared withtwo other films: film A and film B.

In FIG. 9, the transmittance of the spectrally selective solar controlwindow film according to the present prevention is given by line 92; thetransmittance of film A is given by line 94 and the transmittance offilm B is given by line 96.

In FIG. 10, the reflectance of the spectrally selective solar controlwindow film according to the present prevention is given by line 102;the reflectance of film A is given by line 104 and the reflectance offilm B is given by line 106.

Film A comprises alternating layers of In₂O₃ and of AgAu:

In₂O₃ layer/AgAu alloy layer/In₂O₃ layer/AgAu alloy layer/In₂O₃layer/AgAu alloy layer/In₂O₃ layer.

Film B comprises alternating layers of SnO₂ and Ag:

SnO₂ layer/Ag layer/SnO₂ layer/Ag layer/SnO₂ layer.

From FIG. 9, it can be concluded that the visual light transmittance(VLT) of the structure according to the present invention is almostequal to the VLT of film A.

This means that for the structure according to the present invention thedesired VLT can be obtained with only two silver containing layers,whereas the structure of film A needs three silver containing layers.

From FIG. 10, it can be concluded that the reflectance of the infra-redof the structure according to the present invention is higher than thereflectance of the infra-red of the structure of film B.

1. A layered structure comprising consecutively: an infra-red reflectinglayered structure, said infra-red reflecting layered structurecomprising: a first transparent substrate layer; a first metal oxidelayer; a first silver containing layer; a second metal oxide layer; asecond silver containing layer; a third metal oxide layer; a firstadhesive layer; a second transparent substrate layer; a second adhesivelayer; and a glass substrate, wherein said infra-red reflecting layeredstructure further comprises at least one protective intermediate layercomprising gold, said protective intermediate layer being located onboth sides of at least one of the first and second silver containinglayers; said first, second and third metal oxide layer having arefractive index of at least 2.40 at a wavelength of 500 nm and saidinfra-red reflecting layered structure, having a visual lighttransmittance (VLT) higher than 70% and a solar heat gain coefficient(SHGC) lower than 0.44.
 2. A layered structure according to claim 1,wherein said infra-red reflecting layered structure has a light to solargain ratio (LSG ratio) higher than 1.60.
 3. A layered structureaccording to claim 1, wherein said first, second and third metal oxidelayer comprises TiO₂.
 4. A layered structure according to claim 3,wherein said TiO₂ is mainly composed of rutile phase.
 5. A layeredstructure according to claim 1, wherein said first and second silvercontaining layer have a thickness between 10 and 25 nm.
 6. A layeredstructure according to claim 1, wherein said first, second and thirdmetal oxide layer have a thickness between 25 and 70 nm.
 7. A layeredstructure according to claim 1, wherein the infra-red reflecting layeredstructure is a transparent heat-mirror.
 8. A layered structurecomprising consecutively: an infra-red reflecting layered structure,said infra-red reflecting layered structure comprising: a firsttransparent substrate layer; a first metal oxide layer; a first silvercontaining layer; a second metal oxide layer; a second silver containinglayer; a third metal oxide layer; a first adhesive layer; a secondtransparent substrate layer; a second adhesive layer; and a glasssubstrate, wherein said infra-red reflecting layered structure furthercomprises at least one protective intermediate layer comprising gold,said protective intermediate layer being located between a silvercontaining layer and a metal oxide layer and/or between a metal oxidelayer and a silver containing layer; wherein said first, second andthird metal oxide layer is titanium dioxide deposited by reactive DCmagnetron sputtering from a substoichimetric TiO_(x) target where x isin the range between 1.5 to 2, and wherein said first, second and thirdmetal oxide layer has a refractive index of at least 2.40 at awavelength of 500 nm and having a visual light transmittance (VLT)higher than 70% and a solar heat gain coefficient (SHGC) lower than0.44.